Lately there has been much public discussion and debate regarding a so-called “value-added model” for teacher evaluation. Also referred to as a “growth” model, the idea behind this method is relatively simple: A teacher’s professional worth is determined by the academic performance (mostly test scores) of her students while they are with her from September to June.

If the students’ performance improves in that time period the teacher will be deemed an “effective” teacher. If not, she’ll be rated “ineffective” and face the consequences of that poor designation.

For most casual observers, as well as its staunchest supporters, this kind of evaluation makes perfect sense. After all, isn’t it a teacher’s responsibility to make sure her students master the material she has presented and are able to demonstrate that by passing some kind of test? And if they can’t pass the test, surely the teacher has failed the students.

For many critics of the value-added model, myself included, the idea described above is, frankly, too simple. The notion that a teacher is only as good as this year’s student test scores ignores the multitude of factors that make up the complex realities of teaching and learning. In many schools, students are taught by a team of teachers. How would credit (or blame) be assigned in a situation like that?

Many schools group students together by ability. How can the teacher with a group of struggling students be fairly compared to the teacher working with a group of high-achieving students? How will the teachers in a school with a large population of students still learning to speak English fare when compared to teachers in another school across town whose students are all fluent? How will teachers of subjects without standardized tests (music, art, phys ed, etc.) be evaluated?

Logistical issues like these must be addressed in order to create a fair — teacher-to-teacher, classroom-to-classroom, school-to-school — evaluation system.

Factors beyond any teacher’s control also have a powerful effect on student learning. As we all know, all schools are not created equal, so things like class size and makeup, availability of support (teaching assistants, tutors, reading specialists, etc.), access to technology and curriculum resources also impact a student’s learning in any given school during any given year. How will teachers in schools or districts with very few resources be compared to those that are well-funded?

Student learning is also influenced by circumstances beyond any school or classroom. Student attendance, parental support, health, nutrition, etc., can have as much impact on student success as any teacher. Under a value-added model, will those who teach students coming to school ready to learn with extensive support networks fare better than those who teach students without? How will the growth model account for these variables?

As a practical matter, the debate regarding value-added teacher evaluation in New York is over. As part of its effort to win a Race To The Top grant (New York’s share is nearly $700 million), the state has already passed legislation requiring the model for all public school teachers. Obviously, teachers are uncomfortable with the possibility of employment decisions being made using a test-score-based evaluation system that fails to answer and account for all of these questions and many more that will arise as this process goes forward.

Having made the decision to implement a value-added model, the state has an obligation to ensure that the multiple measures (not just test scores) are used to evaluate teachers and that the many variables that influence teaching and learning are factored into the equation.